In telecommunication, the transmission path used to relay signals is known to cause interference to the telecommunication. This occurs regardless of the physical form of the transmission path, whether it is e.g. a radio link, an optical fibre or a copper cable. Particularly in radio communication, situations often arise wherein the quality of the transmission path varies from one data link to another and during the link, too.
Fading is a typical phenomenon on the radio path, causing changes to the transmission channel. Other simultaneous links may also cause interference, which may vary as a function of time and place.
A solution to the problem is the use of diversity in the transmitter. Temporal diversity uses interleaving and coding, which achieve temporal diversity in the signal to be transmitted. However, the disadvantage here is that delays are created in the transmission, especially when the channel is slowly fading. In frequency diversity, in turn, the signal is transmitted simultaneously at several frequencies. However, this is an ineffective method when the channel has a wide coherence bandwidth.
Antenna diversity uses more than one antenna in signal transmission and/or reception. In this case, the signal components that multipath propagated through different channels are not likely to be disturbed by a simultaneous fading. In receive diversity, two or more antennas having a different location or polarization are used to receive a transmitted signal. A disadvantage in receive diversity is that the use of two antennas is difficult to implement in a small-sized terminal. In transmit diversity, the same signal is transmitted to the receiver using two or more different antennas. Transmit diversity is more easily applicable in downlink in mobile telephone systems than is receive diversity, since it is easier to provide a base station than a terminal with more than one antenna.
Another way to use several antennas is the MIMO method (multiple input, multiple output). MIMO is described in detail in the publication G. J. Foschini, ‘Layered Space-Time Architecture for Wireless Communication in a Fading Environment When using Multi-Element Antennas’, Bell Labs Technical Journal, Autumn 1996, which is incorporated herein by reference. MIMO can achieve good performance, but it is subject to the signals transmitted and received via different antennas propagating via different channels. In other words, the channels should not correlate much with each other.
The aim in telecommunication is not only to transmit a signal as faultlessly as possible but also to transfer information as efficiently as possible. Herein, efficiency means that the aim is to utilize the capacity of the transmission channel as efficiently as possible in data transmission. The transmission rates to be achieved in planning cellular radio systems are particularly interesting.
Conventionally, the use of diversity and an increase in transmission rate have been mutually exclusive alternatives.